In recent years, attempts have been made in various fields to investigate substances which enhance brain functions such as learning ability and memory. According to these attempts, the brain function-improving methods so far studied are roughly divided into brain energy metabolism-improving methods in which the action of brain cells is activated through efficient absorption of nutriment into the cells and brain circulation improving methods in which brain cells are supplied with sufficient nutrients and oxygen by improving brain blood circulation. These studies are in progress and drugs having respective pathological actions and therapeutic methods using such drugs are being developed.
On the other hand, .alpha.-linolenic acid and docosahexaenoic acid are known as natural food components which have learning function improving effect. It is known that administration of .alpha.-linolenic acid improves the correct response ratio in a rat brightness-discrimination learning test (Seikagaku (Biochemistry), vol.59, p.1235, 1987) and administration of docoeahexaenoic acid improves the same in a rat Y labyrinthine light and shadow discrimination feeding test (JP-A-1-279827; the term "JP-A" as used herein means an "unexamined published Japanese patent application").
From the standpoint, of the positive incorporation of functions of natural food components, social concern has been directed in recent years toward the development of food products which are designed and processed in such a fashion that their functions can be expressed fully in the living body. Also, because of the increasing aged population, great concern has been directed increasingly toward the development of physiologically functional foods having such functions.